feat: implement single POST upsert API with automatic ID generation

- Replace separate POST/PUT endpoints with unified POST upsert
- Add automatic content ID generation from element context when no ID provided
- Implement version history preservation before content updates
- Add element context support for backend ID generation
- Update frontend to use single endpoint for all content operations
- Enhanced demo site with latest database content including proper content IDs

demo-site/about.html

@@ -9,7 +9,7 @@
  <!-- Navigation -->
  <nav class="navbar">
  <div class="container">
- <h1 class="logo insertr" >Acme Consulting</h1>
+ <h1 class="logo insertr" data-content-id="navbar-logo-2b10ad" data-content-type="text">Acme Consulting</h1>
  <ul class="nav-links">
  <li><a href="index.html">Home</a></li>
  <li><a href="about.html">About</a></li>
@@ -22,15 +22,15 @@
  <!-- Hero Section -->
  <section class="hero">
  <div class="container">
- <h1 class="insertr" >About Acme Consulting</h1>
- <p class="lead insertr" >We&#39;re a team of experienced consultants dedicated to helping small businesses thrive in today&#39;s competitive marketplace.</p>
+ <h1 class="insertr" data-content-id="hero-title-c70343" data-content-type="text">About Acme Consulting</h1>
+ <p class="lead insertr" data-content-id="hero-lead-673026" data-content-type="markdown">We&#39;re a team of experienced consultants dedicated to helping small businesses thrive in today&#39;s competitive marketplace.</p>
  </div>
  </section>
 
  <!-- Story Section -->
  <section class="services">
  <div class="container">
- <h2 class="insertr" >Our Story</h2>
+ <h2 class="insertr" data-content-id="services-subtitle-c8927c" data-content-type="text">Our Story</h2>
  <div class="insertr-group">
  <p>Founded in 2020, Acme Consulting emerged from a simple observation: small businesses needed access to the same high-quality strategic advice that large corporations receive, but in a format that was accessible, affordable, and actionable.</p>
  
@@ -44,8 +44,8 @@
  <!-- Team Section -->
  <section class="cta">
  <div class="container">
- <h2 class="insertr" >Our Team</h2>
- <p class="insertr" >We&#39;re a diverse group of strategists, operators, and technology experts united by our passion for helping businesses succeed.</p>
+ <h2 class="insertr" data-content-id="subtitle-ba6444" data-content-type="text">Our Team</h2>
+ <p class="insertr" data-content-id="text-8b3502" data-content-type="markdown">We&#39;re a diverse group of strategists, operators, and technology experts united by our passion for helping businesses succeed.</p>
  
  <div class="services-grid" style="margin-top: 3rem;">
  <div class="service-card">
@@ -76,19 +76,19 @@
  <!-- Values Section -->
  <section class="testimonial">
  <div class="container">
- <h2 class="insertr" style="margin-bottom: 2rem;" >Our Values</h2>
+ <h2 class="insertr" style="margin-bottom: 2rem;" data-content-id="testimonial-subtitle-dce35b" data-content-type="text">Our Values</h2>
  <div style="display: grid; grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)); gap: 2rem; text-align: left;">
  <div>
- <h3 class="insertr" >Client-First</h3>
- <p class="insertr" >Every recommendation we make is designed with your specific business context and goals in mind.</p>
+ <h3 class="insertr" data-content-id="testimonial-heading-f7eefa" data-content-type="text">Client-First</h3>
+ <p class="insertr" data-content-id="testimonial-text-f7e46f" data-content-type="markdown">Every recommendation we make is designed with your specific business context and goals in mind.</p>
  </div>
  <div>
- <h3 class="insertr" >Practical Solutions</h3>
- <p class="insertr" >We believe in strategies that you can actually implement with your current resources and capabilities.</p>
+ <h3 class="insertr" data-content-id="testimonial-heading-fd4293" data-content-type="text">Practical Solutions</h3>
+ <p class="insertr" data-content-id="testimonial-text-c12023" data-content-type="markdown">We believe in strategies that you can actually implement with your current resources and capabilities.</p>
  </div>
  <div>
- <h3 class="insertr" >Long-term Partnership</h3>
- <p class="insertr" >We&#39;re not just consultants; we&#39;re partners in your business success for the long haul.</p>
+ <h3 class="insertr" data-content-id="testimonial-heading-957cc8" data-content-type="text">Long-term Partnership</h3>
+ <p class="insertr" data-content-id="testimonial-text-45dae2" data-content-type="markdown">We&#39;re not just consultants; we&#39;re partners in your business success for the long haul.</p>
  </div>
  </div>
  </div>
@@ -97,7 +97,7 @@
  <!-- Test Section for Insertr Features -->
  <section class="testimonial">
  <div class="container">
- <h2 class="insertr" >Feature Tests</h2>
+ <h2 class="insertr" data-content-id="testimonial-subtitle-648b88" data-content-type="text">Feature Tests</h2>
  
  <!-- Test 1: .insertr container expansion (should make each p individually editable) -->
  <div style="margin-bottom: 2rem;">
@@ -124,8 +124,8 @@
  <!-- Footer -->
  <footer class="footer">
  <div class="container">
- <p class="insertr" >© 2024 Acme Consulting Services. All rights reserved.</p>
- <p class="insertr" >📧 info@acmeconsulting.com | 📞 (555) 123-4567 | <button class="insertr-gate" style="background: none; border: 1px solid #ccc; padding: 4px 8px; margin-left: 10px; border-radius: 3px; font-size: 11px;">🔧 Edit</button></p>
+ <p class="insertr" data-content-id="footer-text-a2b6a8" data-content-type="markdown">© 2024 Acme Consulting Services. All rights reserved.</p>
+ <p class="insertr" data-content-id="footer-text-a44170" data-content-type="markdown">📧 info@acmeconsulting.com | 📞 (555) 123-4567 | <button class="insertr-gate" style="background: none; border: 1px solid #ccc; padding: 4px 8px; margin-left: 10px; border-radius: 3px; font-size: 11px;">🔧 Edit</button></p>
  </div>
  </footer>